It has been found that amplitude and delay distortion resulting from multipath fading can, under certain conditions, be a major cause of transmission deterioration in both digital and FM radio communications systems. Experimental and analytical evidence indicate that outages of wideband digital systems, caused by this phenomenon (i.e., frequency selective fading), may exceed system performance objectives. This means that techniques for handling flat fades, such as AGC, will not be adequate to maintain satisfactory wideband digital transmission, and that additional correction will be required. Even the use of space diversity techniques will not eliminate completely the problems associated with selective fading. The use of some form of equalization will still be required to achieve the desired level of system performance. (See, for example, U.S. Pat. No. 4,261,056, filed July 16, 1979, and assigned to applicant's assignee.)
The problem, however, is that signal fading resulting from multipath transmission is basically unpredictable. Accordingly, the compensation introduced by a fade equalizer must be capable of automatically adapting to the changing signal conditions. One such adaptive equalizer, employing feedback techniques, is disclosed by H. Miedema in his copending application, Ser. No. 158,404, filed June 11, 1980, now U.S. Pat. No. 4,330,764. While this equalizer compensates the amplitude distortion, it does not provide delay equalization in the case of nonminimum phase fades. Indeed, for a nonminimum phase fade, the delay distortion is doubled. In another variation of the feedback equalizer, disclosed in a copending application by G. D. Martin, Ser. No. 203,645, filed Nov. 3, 1980 now U.S. Pat. No. 4,361,892, all-pass networks, which are more difficult to realize, are used to compensate the delay distortion for both minimum and nonminimum phase fades.